SIP协议之匿名呼叫
匿名呼叫是SIP呼叫中的一个特殊业务，原理是构造一个没有个人信息的From头域，其中显示名部分为Anonymous, 这样被叫收到呼入请求时，按照From头域中的显示部分“Anonymous"显显示为Anonymous（匿名）来电。
消息示例如下：
INVITE sip:7333@192.168.0.108:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.6:5060;branch=z9hG4bKfdf518ac1bf22845c7a7a
From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=657128773
To: <sip:7333@192.168.0.108:5060>
Call-ID: 1_657143602@192.168.1.6
CSeq: 1 INVITE
Contact: <sip:9223@192.168.1.6:5060>
Content-Type: application/sdp
Allow: INVITE, INFO, PRACK, ACK, BYE, CANCEL, OPTIONS, NOTIFY, REGISTER, SUBSCRIBE, REFER, PUBLISH, UPDATE, MESSAGE
Max-Forwards: 70
User-Agent: Yealink SIP VP-T8G
Privacy: id
Allow-Events: talk,hold,conference,refer,check-sync
P-Preferred-Identity: <sip:7223@192.168.0.108>
Supported: replaces
Content-Length: 200xxxxxx